Retroreflective materials are employed for various safety and decorative purposes. Particularly, these materials are-useful at night time when visibility is important under low light conditions. With perfect retroreflective materials, light rays are reflected essentially towards a light source in a substantially parallel path along an axis of retroreflectivity. For many applications, perfect retroreflectivity is not required. Rather, a compromise is required in which a cone of divergence is provided which permits a degree of divergence which enables enough divergent light to strike the viewer's eye, yet not so much that the intensity of the reflective light at the viewer's eye is unduly diminished. Under circumstances where the only source of illumination is the headlights of an automobile on an unlit road, the ability to retroreflect a cone of divergence to the eye of the driver is important for safety reasons.
Many types of retroreflective material exist for various purposes. These retroreflective materials can be used as reflective tapes and patches for clothing, such as vests and belts. Also, retroreflective bands can be used on posts, barrels, traffic cone collars, highway signs, warning reflectors, etc. Retroreflective material may be comprised of arrays of randomly oriented micron diameter spheres or close packed cube-corner (prismatic) arrays.
Cube-corner or prismatic retroreflectors are described in U.S. Pat. No. 3,712,706, issued to Stamm on Jan. 23, 1973. Generally, the prisms are made by forming a master negative die on a flat surface of a metal plate or other suitable material. To form the cube-corners, three series of parallel equidistance intersecting V-shaped grooves 60 degrees apart are inscribed in the flat plate. The die is then used to process the desired cube-corner array into a rigid flat plastic surface.
When the groove angle is 70 degrees, 31 minutes, 43.6 seconds, the angle formed by the intersection of two cube faces (the dihedral angle) is 90 degrees and the incident light is reflected back to the source. For automobile headlight reflectors, the dihedral angle is changed so that the incidental light is reflected nonorthogonally towards the driver instead of the source.
The efficiency of a retroreflective structure is a measure of the amount of incidental light returned within a cone diverging from the axis of retroreflection. Distortion of the prismatic structure adversely effects the efficiency. For instance, if the prismatic structure is formed of a thermoplastic, the structure can distort if it is overstressed, thereby decreasing the efficiency of the retroreflective structure. One solution is to form a prismatic structure with a hard polymer. However, if the support sheeting is formed of a thermoplastic, a suitable weld is difficult to form between the thermoplastic sheets, thereby allowing the formed structure to tear easily along the weld. Furthermore, cube-corner retroreflective elements have low angularity, i.e., the element will only brightly retroreflect light that impinges on it within a narrow angular range centering approximately on its optical axis. Low angularity arises by the inherent nature of these elements, which are trihedral structures having three mutually perpendicular lateral faces. The elements are arranged so that light to be retroreflected impinges into the internal space defined by the faces, and retroreflection of the impinging light occurs by internal reflection of the light from face to face of the element. Impinging light that is inclined substantially away from the optical axis of the element (which is the trisector of the internal space defined by the faces of the element) strikes a face at an angle less than its critical angle, thereby passing through the face rather than being reflected.
Further details concerning the structures and operation of cube-corner microprisms can be found in U.S. Pat. No. 3,684,348, issued to Rowland on Aug. 15, 1972, the teachings of which are incorporated by reference herein. A method for making retroreflective sheeting is also disclosed in U.S. Pat. No. 3,689,346, issued to Rowland on Sep. 5, 1972, the teachings of which are incorporated by reference herein. The disclosed method is for forming cube-corner microprisms in a cooperatively configured mold. The prisms are bonded to sheeting which is applied thereover to provide a composite structure in which the cube-corner formations project from one surface of the sheeting.